(1) Technical Field
This invention generally relates to electronic circuitry, and more specifically to tunable integrated impedance matching networks for use in electronic radio frequency circuits.
(2) Background
Radio frequency (RF) communication systems typically include “RF front-end” (RFFE) circuitry, which is a generic term for all of the circuitry between a radio antenna up to and including the mixer stage of a radio. Impedance matching (IM) networks are an essential building block in RFFE circuits in order to match the internal impedance of an RFFE (e.g., 2-3 ohms) to the nominal impedance value (typically 50 ohms) of the characteristic impedance of common RF systems.
FIG. 1 is a schematic diagram of an RF front end 100 including one type of prior art impedance matching network 102. The IM network 102 is shown connected to a power amplifier 104 and an antenna 106. The illustrated IM network 102 includes two series-connected stages of inductors L1, L2, and corresponding shunt capacitors C1, C2 connected to circuit ground as shown. In other embodiments, more stages of inductors and shunt capacitors may be used. The values selected for L1, L2, C1, and C2 are design and system dependent, but the function and design of such IM networks 102 is well known in the art.
When implemented with integrated circuit technology, the inductors used in typical RFFE's are commonly formed as 2-port spirals; examples of such spiral inductors are shown in U.S. Pat. No. 5,656,849. FIG. 2 is a schematic diagram showing one type of prior art impedance matching network 200 having spiral inductors L1s, L2s shown in place of the inductors L1, L2 of FIG. 1; the connection to the center port or tap of the spiral inductors L1s, L2s is shown as a dotted line. Note that the spiral inductors L1s, L2s are shown highly stylized as essentially Archimedean spirals; however, such inductors may be of various overall shapes and sizes so long as inductive loops (in the broadest sense) are formed, as is known in the art. For example, U.S. Pat. No. 5,656,849 illustrates spiral inductors with straight sides and right angle turns.
RF circuitry implemented in low-power integrated circuit (IC) technologies (“chips”) such as CMOS has enabled increasingly more compact radio systems, such as cell phones and other RF-connected mobile devices. However, while transistors continue to shrink in size as fabrication process technology advances, passive devices such as inductors have not scaled down at the same rate. In particular, on-chip inductors used in RF circuits tend to be the most area-consuming devices, especially since the layout area of IC inductors is not determined by the feature size of an implementing CMOS process but rather upon such factors as the RF carrier frequency, the data rate of the circuit, desired and sufficient inductor Q factor, etc. Further, each inductor often requires long hours of optimization time. Multiband RF front-end circuitry typically uses many inductors to obtain wideband characteristics, resulting in large IC chip (die) sizes and large expenditures of optimization time. Notably, a large IC die size produce smaller yields per semiconductor wafer than smaller IC dies, thus increasing the production cost per die.
Another problem with inductors is that of mutual inductive coupling, in which the flow of current in one inductor (e.g., L1 or L1s) induces a voltage in a nearby inductor (e.g., L2 or L2s), and vice versa. Conventional designs generally try to minimize inductive coupling by physically spacing inductors apart (which often leads to larger IC chip sizes), or by designing the inductors to destructively couple (i.e., have currents flow in opposite directions) to minimize the adverse effects of mutual inductance. In spiral conductors, destructive coupling is generally accomplished by having adjacent inductors wind in opposite directions, as suggested by the winding direction of the spiral inductors L1s and L2s in FIG. 2.
It would be desirable to be able to reduce the size of inductor-based IM matching networks to shrink overall circuit size and reduce production costs, while maintaining high performance and without the adverse effects of mutual inductive coupling as may occur in conventional designs. The present invention accomplishes these goals.